The main goal of the proposed research is to elucidate the nature and mechanism of the changes in the physical properties of the collagenous articular capsule, or ligament, that surrounds the spine-test joint of the primary spines of the sea urchin Eucidaris tribuloides. Under the influence of the nervous system the consistency of this ligament can change rapidly and reversibly between two extreme conditions: soft, i.e. extensible, and stiff, or inextensible. Acetylcholine (ACh) consistently stiffens the ligament and may be regarded as the putative catch-inducing neurotransmitter. Pharmacological techniques will be used to search for neurotransmitters or drugs which may induce simple softening and to describe their interactions with ACh. Electron microscopy of conventional preparations and of material stained with cuprolinic blue will be used to characterize cross-links between collagen fibrils and columns and to detect possible alterations in the arrangement of the proteoglycans associated with changes in mechanical properties. In addition, histochemical techniques, using fluorescent phalloidin, anti- actin antibodies and the decoration of actin filaments with heavy meromyosin, will be employed to test the possible presence of actin filaments in the ligament matrix.